VORTEX MERGER ON THE BETA-PLANE

We investigate numerically the influence of the planetary vorticity gr adient (beta-effect) on the merger of two identical vortices, in a mod el of a two-dimensional incompressible inviscid flow. On each vortex, the beta-effect induces a drift velocity and an energy dispersion into Rossby waves. First, we define the boundary in beta between the wave and the vortex regimes. In the absence of beta, the two vortices under go the well-known transition between corotation and merger when we var y the initial intercentroid distance (normalized by the vortex radius) d/R. But the anisotropic beta-effect induces an asymmetry in the vort ex configuration with time, and renders the problem similar to that of asymmetric vortex merger without beta. The nonlinear evolutions are t hus classified in the (beta, d/R) parameter space into complete merger , partial merger, partial straining-out, and two elastic interactions, corotation and differential drift.